Make clojure-find-ns handle ns declarations with leading whitespace

`clojure-find-ns` does not currently find a namespace if the source file contains leading spaces before the `ns` form:

```clojure
     (ns blahonga)
; ^- whitespace, oh no!

```
Compiling this file, and then trying to switch to the namespace will fail.

This pull request attempts to solve this (very minor but subtle) issue by extending `clojure-namespace-name-regex` to allow zero or more whitespace characters before the ns form